The present invention relates to a method for treating an exhaust gas, in particular a dioxin-containing exhaust gas emitted from a waste incinerator.
FIG. 1 is a diagram illustrating a typical method for removing particulate matter (e.g., flyash) from exhaust gas produced by a waste incinerator. Burning waste (e.g., municipal waste) in an incinerator creates byproducts of (i) ash and (ii) exhaust gas and flyash, the former residing in the incinerator itself and the latter passing through the stack of the incinerator. It is standard operating procedure to flow the exhaust gas and flyash through a boiler to quench the exhaust gas and reduce the temperature thereof to a sufficiently low level so that a bag filter can be used to remove the flyash from the exhaust gas. The resultant exhaust gas is then passed through a scrubber and emitted to the environment through a stack.
It is well known that the incineration of municipal waste materials creates large volumes of organic compounds and hydrocarbons. These materials serve as precursors for various compounds, some of which are highly toxic. For example, aromatic compounds such as phenol or benzene, or chlorinated aromatic compounds such as chlorophenol or chlorobenzene, react in the presence of flyash to form dioxin, which is highly toxic.
It is believed that formation of dioxin in the presence of flyash is the result of a catalytic reaction wherein flyash is the catalyst. It is also believed that the catalytic reaction occurs when the temperature of the exhaust gas drops below 400.degree. C., which typically occurs at a location between the boiler and the bag filter.
While it would seem logical to simply remove the flyash from the exhaust gas before the temperature of the exhaust gas drops below 400.degree. C., and thus prevent the formation of dioxin in the first instance, there is no industrially practical method or apparatus for accomplishing such a goal. Accordingly, the industry has adopted various methods by which dioxin is removed from incinerator exhaust gas prior to being emitted to the environment through the stack of the incinerator.
The use of sorbent materials is the most common method for removing dioxin from incinerator exhaust gas. Sorbents are materials that adsorb or absorb dioxin or dioxin precursors, and examples of such sorbents include certain cements (JP 97-2678543), activated carbon and activated white clay (JP 92-87624 A and JP 96-243341 A), activated coke (JP 97-29046 A), silicates (JP 97-75719 A and JP 97-75667 A), and zeolites (JP 97-248425 A).
While it is most common to add such sorbents to the exhaust gas at an exhaust gas temperature of less than 400.degree. C., to thereby sorb dioxin per se, another known method (EP 0 764 457) discloses adding sorbents to the exhaust gas at an exhaust temperature of greater than 400.degree. C. to remove dioxin precursors from the exhaust gas.
While all of the above-described methods are effective to remove dioxin from the exhaust gas to some degree, there are problems associated with each method. The main problem with using carbon-based sorbents is that there is a distinct possibility that the carbon will oxidize in the exhaust stream and cause a fire in the bag filter, for example. In addition to the obvious danger associated with such a fire, the heat generated as a result of the fire would cause all of the dioxin or dioxin precursors sorbed on the activated carbon to desorb and thus be emitted out of the incinerator stack.
The problem with using other sorbents such as silicates and zeolites, for example, is that the desorption temperature of those materials is too close to the vaporization temperature of dioxin itself. Specifically, the vaporization temperature of dioxin is about 220.degree. C., whereas the temperature at which dioxin desorbs from materials such as silicates and zeolites ranges from about 220.degree. C. to 260.degree. C. Sorption of dioxin is most effective when the dioxin is in a gaseous state, and the sorption efficiency of a sorbent depends largely upon how close the dioxin desorption temperature of the material is to the vaporization temperature of dioxin. Accordingly, the sorption efficiency of materials such as silicates and zeolites is relatively poor, because the desorption temperature of those materials is too close to the vaporization temperature of dioxin.
One reason that activated carbon is effective as a dioxin sorbent is that its desorption temperature is not close to the vaporization temperature of dioxin. The problem of combustion in the bag filter, however, is still a significant concern.
It would be desirable to provide a method for removing dioxin from incinerator exhaust gases without the threat of fire (associated with the use of activated carbon) and without the problem of sorption inefficiency (associated with materials such as silicates and zeolites). To date, however, the industry has not provided any such method.